1. Field of the Invention
The present invention relates to a process for core-shell emulsion polymers. More specifically the invention relates to a process for hydrophobically modified core-shell emulsion polymers.
2. Background of the Invention
Emulsion polymers may be formed by either batch one step, or by two step emulsion polymerization processes. The batch process refers to combining all reactants and polymerizing in a single step. A two step emulsion polymerization refers to forming a first polymer and then polymerizing additional reactants in the presence of the first polymer to form the final emulsion polymer. The two step emulsion polymerization process may be used to form a core-shell polymer.
The core-shell polymer refers to the morphology of the final polymer particles. The core refers to the polymer formed in the first step. The shell refers to the polymer formed in a second step by polymerizing additional monomers in the presence of the polymer formed in the first step. The monomers for the core may be varied from those of the shell to provide a core having varied characteristics from the shell. Such variations include differing hardnesses by using monomers with different glass transition temperatures, as taught by Morgan et al. in Journal of Applied Polymer Science, Vol. 27, p. 2033-2042 (1982). Other variations include varying the polarity or varying molecular weights of the core and shell.
Emulsion polymers are desirable for use in coating compositions, as these provide waterborne coatings with lower volatile organic content than solvent borne coatings. Such emulsions when used in a coating provide excellent appearance and desirable rheological characteristics.
Core-shell emulsion polymers containing a hydrophobic component are highly useful when used in a coating composition. In addition to the benefits listed above for emulsion polymers, the core-shell emulsion polymers contribute to improved solvent tolerance of the coating composition. Hydrophobic core-shell polymers can be used to provide high solids waterborne coatings. The hydrophobicity is generally introduced via acrylic monomers having at least four carbon atoms in the side chain. However, the level of hydrophobicity which can be introduced by such methods is limited.
It is desirable, therefore, to provide a process for producing core-shell emulsion polymers having a high concentration of hydrophobe, and whereby a variety of hydrophobes can be introduced.